Add doc on testing (#23)

Signed-off-by: i2y <6240399+i2y@users.noreply.github.com>
Signed-off-by: Yasushi Itoh <i2y@users.noreply.github.com>
Co-authored-by: i2y <6240399+i2y@users.noreply.github.com>
Co-authored-by: Anuraag (Rag) Agrawal <anuraaga@gmail.com>

Author: i2y